Conventionally, there is known a system that feeds pressure oil discharged from a fixed-capacity pump to a high-pressure circuit of a clutch or the like, depressurizes and feeds the pressure oil of the high-pressure circuit using a relief valve to a low-pressure circuit of a lubrication circuit or the like (e.g., see FIG. 2 of Japanese Laid-open Utility Model Application No. 5-2742 as an illustration of prior art).
Such a conventional system will be described with reference to FIG. 7. This conventional oil pressure system 200 is provided with an engine 201, an oil pressure pump 202, a first flow passage 203, a relief flow passage 204, and a relief valve 205. The oil pressure pump 202 is driven by the engine 201, and the discharge rate of the pressure oil is varied in accordance with the rotational speed of the engine 201. The first flow passage 203 is connected to a predetermined high-pressure circuit and is a flow passage for feeding pressure oil discharged from the oil pressure pump 202 to the high-pressure circuit. The relief flow passage 204 is a flow passage for connecting the first flow passage 203 and the low-pressure circuit. The relief valve 205 closes the relief flow passage 204 when the pressure of the pressure oil that flows through the first flow passage 203 is at a predetermined relief pressure or less, and opens the relief flow passage 204 when the pressure of the pressure oil that flows through the first flow passage 203 is greater than the relief pressure. In other words, the relief valve 205 has the role of feeding pressure oil to the low-pressure circuit while compensating the pressure of the high-pressure circuit.
In this configuration, the oil pressure pump 202 is a fixed-capacity pump, and the flow rate of the pressure oil fed to the first flow passage 203 is therefore increased in accompaniment with the increased rotational speed of the engine 201, as shown in FIG. 8(b). The oil pressure pump 202 is selected in accordance with a specification that allows a required oil quantity Q1 to be supplied when the engine 201 is rotating at a low-idle rotational speed N1. FIG. 8(b) shows the relationship between the engine rotational speed and the flow rate of the pressure oil fed to the high-pressure circuit via the first flow passage 203.
Generally, there are many cases in which the low-pressure circuit requires a greater quantity of oil than does the high-pressure circuit. In the conventional system described above, the loss of power is considerable because the pressure oil is fed to the low-pressure circuit via the relief valve 205, which is constantly set to a high relief pressure.
In order to solve this problem, oil pressure systems such as that shown in Japanese Laid-open Utility Model Application No. 5-2742 have been proposed in which there are used a low-capacity high-pressure pump for a high-pressure circuit and a high-capacity low-pressure pump for a low-pressure circuit. In the art of Japanese Laid-open Utility Model Application No. 5-2742, a technique is disclosed in which a priority valve is provided between the low-pressure circuit and the clutch circuit, which is a high-pressure circuit, and the priority valve is operated during clutch operation to feed the discharge oil of the high-capacity low-pressure pump to the high-pressure circuit side.